Method of making weathering corrections



a o J i H. c. SCHAEFFER mm -fi a METHOD OF MAKING WEATHERING CORRECTIQNSFiled July 28, 1939 R a. PE TERsoM INVENTORS JCHAEFFE Hue/ '6 A a qp 7am; Ar us/51 s.

- PATENT GFFi attain METHOD F 7 WEATMRHNG WRREUEKGNS iilugh EC.Schaefier, Tulane, and Raymond A.

Peterson, Aitadena, Salim, assignors to Consolidated EngineeringCorporation, Pasadena, Calif., a corporation of California ApplicationJuly 2%, 1939, Serial No. 287,075

6 Claims. (m. 1181-05) Our invention pertains to seismic prospecting bydetonating a charge of explosive E at the and in particular provides amethod for deterbottom of shot hole S2 and the waves generated miningthe time required'for waves to travel subare detected by a group ofreceptors R6 to R11 stantially Vertically through the weathered layer.straddling said shot hole S2 and corresponding and also the thickness ofthe weathered layer electrical waves are transmitted over cables to 5beneath a seismic wave receptor. a muiti-element recording galvanometerG, en-

In conducting seismic exploration, waves are ergized by the usualamplifiers. ordinarily received at a plurality of receptors In thearrangement shown, the first waves to I spaced on the surface of theearth. Due to varireach receptors R6 and R11 from the charge E 10 ationsin the thickness of the weathered layer, are those which travel overpaths in and 2| 10 or low speed layer at the surface, the time repartlythrough the high speed layer 2 and partly quired for waves to travelthrough said weaththrough the low speed layer l. At a later time, eredlayer vary from one receiving position to waves reflected from bed 3 arereceived at re-' another. In areas where accurate dip deterceptors Reand R11 after being transmitted over 5 minations are required it isnecessary to be able path; 22 and 23. Reflected waves are received 15 tocorrect for such difierences in the thickness by all receptors Re to R11after reflection from of the weathered layer. By means of our inventheportion M of bed- 3. tion accurate dip determinations become possi- Byshooting at the bottom of shot hole S1 and bio. recording waves receivedat receptors R1 to Re The principal object of our invention is to protheportion L of bed 3 may be investigated. Simvide a method for makingweathering correcilarly the portion N of bed 3 may also be intionsespecially in areas in which a low speed 'vestigated by shooting at thebottom of shot layer overlies a high speed layer. hole S3 and recordingwaves received by re- Our invention possesses numerous other obceptorsR11 to R16- Dotted lines joining L, M,

-jects and features of advantage, some of which, and N signify portionsof bed 3 which are not 25 together with the foregoing, will be set forthin investigated directly from the procedure outlined the followingdescription of specific apparatus but which may be inferred by methodsof corembodying and utilizing our novel method. It is relation.

therefore to be understood that our method is The dip of bed 3 of theportion M is given applicable to other apparatus, and that we doapproximately by the formula 30 not limit ourselves, in any way to theapparatus. VAT of the present application, as we may adopt vara= i (1approx. (1)

ions other apparatus embodiments, utilizing the method, within the scopeof the appended claims. where The drawing represents an isometric viewin- V=speed of waves in high velocity medium 2. 35 eluding a verticalsection of the earth and one X=distance between end receptors R6 andR11. arrangement of apparatus to which our inven- AT=(t2ats)-(t2atu)tion is especially applicable. where In the drawing a weathered layer 8having a tzz=time required for waves to travel from 40 substantiallyuniform low velocity overlies a high shot E to receptor Rs along path22. 40

speed subweathered layer 2 which in turn overtza=time required for wavesto travel from lies a reflecting bed 3. A line of exploration is shot atE to receptor R11 along path 23. p I laid out along the axis X-X. Alongthis line tc=tiine required for a wave to travel along a of explorationshot holes S1, S2, and S: are drilled substantially vertical paththrough the 43 to points beneath the bottom of the weathered weatheredlayer beneath receptor Rs. 45

. layer. Receptors adapted to convert received t11=time required for awave to travel'along seismic waves into corresponding electrical waves asubstantially vertical path through the are shown as R1 to R16 laid outsuccessively along weathered layer beneath receptor R11.

theline of exploration X-X. =the angle between a horizontal line and a 0In one method of investigating the structure line joining points 121 andp2 beneath rebeneath the earths surface for the purposes of ceptors R0and R11 and at the bottom of the determining favorable oil welllocations, the dip weathered layer. of reflecting beds is determined atvarious po- On applying Equation 1 appropriate sign ,sitions along aline of exploration. conventions for a and 9 should be adopted to In thearrangement shown a record is made avoid errors. 55

It will be clear to'those skilled in the art that the value of a. may bedetermined from the relative elevation of the end receptors R6 and R11,the distance between said end receptors and the thickness of theweathered layer beneath said end receptors.

Our problem is to provide a method for making weathering corrections bydetermining the thickness of the weathered layer and the time requiredfor Waves to travel therethrough at end .receptors Re and R11.

As an example, to make weathering corrections for the weathered layerbeneath the receptor R11 we proceed as follows.

Prior to making a recording of the shot E at receptors Re to R11 weplace an auxiliary receptor Rm at the bottom of shot hole S: and connectit to an auxiliary recording unit of multielement recording galvanometerG.

When charge E is detonated by operation of a blaster (not shown), werecord the instant of detonation of said charge E on one of the units ofrecording galvanometer G.

The waves traversing the refraction paths 2!] and 2| and the reflectionpaths 22 and 23 and other waves received by the group of receptors Re toR11 are recorded as hereinabove explained. In addition a recording ismade of the waves traversing the path 40 and received at R11. In asimilar manner we subsequently set up the apparatus to shoot at thebottom of shot holes S3 and record at receptors R11 and Rm.

, From the aforementioned records we are able to determine the followingtravel times.

t21=time required for a wave to travel from a charge E at the bottom ofshot holes S2 to receptor R11 along path 2|.

tai=time required for a wave to travel from a similar charge at thebottom of shot hole S: to receptor R11 along path 3|.

t11=time required for a wave to travel fromthe bottom of shot hole S2 tothe bottom of shot hole S: along a direct path 40 through the high speedlayer 2.

It can be shown readily that the time tu required for waves to travelsubstantially vertically through the weathered layer beneath receptorR11 is given approximately by the formula where V: is the velocity ofseismic waves in the high speed layer 2 which may be determined ifdesired from tm and the distance between shot holes S2 and S3; and whereV1 is the velocity of seismic waves in the low speed layer I. V1 neednot be known exactly for this purpose as it enters into Formula 2 onlyas a correction factor. However, in many areas V1 is substantiallyconstant and may accordingly be determined once and for all by anyspecial method useful for this purpose.

It is to be noted that Equation 2 may be rewritten as which means thatit is not necessary to know both tm and tau but only the differencebetween them.

Having determined tn, the thickness du of the weathered layer beneathreceptor R11 may be found from the relation By making similarmeasurements for receptor R6 such as by shooting at the bottom of shothole S1 and receiving at receptors R1 to Re and a receptor at the bottomof shot hole S2 in accordance with the procedure outlined above, we areable to obtain the time is the time required for waves to travel throughthe weathered layer beneath receptor Rs and the depth of the weatheredlayer beneath receptor Rs.

It is clear that by means of the weathering data obtained for receptorsRe and R11 accurate determinations of the dip of the portion M of bed 3may be obtained.

While we have described our invention with reference to a particulararrangement 'of seismic prospecting apparatus it is to be understoodthat we do not intend to be limited to the details of the description asvarious forms of our inven tion may be adopted within the scope of theappended claims. In particular it is to be understood that the receptor,for which weathering corrections are desired, need not lie on a straightline joining the shot holes used therewith, but may be offset therefromany amount which does not introduce appreciable error. In case theoffset distances are not negligible, suitable correction factors may bemade therefor.

Broadly speaking our invention provides for making weatheringcorrections at a receptor p0- sitiion intermediate two shot holesdrilled to points adjacent the bottom of the weathered layer andsubstantially in line with said shot holes. For this purpose we mayrecord the time for seismic waves to travel from the bottom of each shothole to said receptor position and the time for waves to travel from thebottom of one shot hole to the bottom of the other.

We claim: I

1. A method for making weathering corrections in an area in which a lowspeed surface layer overlies a high speed layer, comprising drilling twospaced shot holes in the earth to depths adjacent the bottom of saidsurface layer, planting a seismic wave receptor station on or adjacentthe earths surface at a point intermediate said shot holes andsubstantially in line therewith and a second seismic wave receptorstation at the bottom of one of said shot holes, generating seismicwaves successively at the bottom of each of said shot holes, measuringthe' time required for a seismic wave to travel from each shot hole tosaid intermediate point, along a path including a portion within andadjacent the top of said high speed layer, and substantially parallelthereto, and measuring the time required for a seismic wave to travelfrom the bottom of one shot hole to the bottom of the other shot holealso over a path including a portion within and adjacent the top of saidhigh speed layer and substantially parallel thereto.

2. A method for making weathering corrections in an area in which a lowspeed surface layer overlies a high speed layer, comprising drilling twospaced shot holes in the earth to depths penetrating the top of saidhigh speed layer, establishing a. seismic wave receptor station on oradjacent the earths surface at a point intermediate said shot holes andsubstantially in line therewith, establishing a seismic wave receptor atthe bottom of one of said shot holes, generating a seismic wave at theother of said shot holes, measuring the relative times required for a.refracted wave to travel to a receptor at said surface station and adirect wave to travelto said receptor at the bottom of said one shothole, generating a seismic wave at the bottom of said one shot hole, andmeasuring the time required for a refracted wave to travel from said oneshot hole to said surface receptor.

3. A method for' making weathering corrections in an area in which a lowspeed surface layer overlies a high speed layer, comprising drilling twospaced shot holes in the earth to penetrate the top of said high speedlayer, establishing a seismic wave receptor station on or adjacent theearths surface at a point intermediate said shot holes and substantiallyin line therewith and a second seismic wave receptor station at thebottom of one of said shot holes, generating seismic waves successivelyat the bottom of each shot hole, measuring the time required for aseismic wave to travel from each shot hole to said intermediate pointalong paths including portions substantially parallel to the top part ofsaid high speed layer, and measuring the time required for a seismicwave to travel from the bottom of one 'shot hole to the bottom of theother shot hole along a direct path through said high speed layer.

4. A method for making weathering corrections in an area in which a lowspeed surface layer overlies a high speed layer, comprising drilling twospaced shot holes in the earth to depths just beneath said surfacelayer, establishing a seismic, wave receptor station'on or adjacent theearths surface at a point intermediate said shot holes and substantiallyin line therewith, establishing a seismic wave receptor at a point atthe bottom of one of said shot holes, generating a seismic wave at thebottom of the other of said shot holes, measuring the relative timesrequired for said refracted wave to travel to a receptor at said surfacestation and a direct wave to travel to a receptor at the bottom of saidone shot hole,

generating a'seismic wave at the bottom of said one shot hole, andmeasuring the time required for a seismic refracted wave to travel fromsaid one shot hole to said surface receptor.

5. A method for obtaining data for making weathering corrections in anarea in which a low speed surface layer overlies a high speed layer,comprising generating a first seismic wave at a generating pointadjacent the earth's surface, measuring the time required for said firstseismic wave to travel from said generating point to a surface receptionpoint along a path including portions within said highspeed layer andsubstantially parallel to the top to travel from said generating pointto a second reception point also along a path including a portion withinsaid high speed layer and substantially parallel to the'top thereof,generating a second seismic wave at one of said reception points andmeasuring the time required for said second seismic wave to travel fromsaid one reception point to the other of said reception points alsoalong a path including a portion within said high speed layer andsubstantially parallel to the top thereof, all said portions of pathslying within said high speed layer being substantially parallel.

6. A method for obtaining data for making weathering corrections in anarea in which a low speed layer overlies a high speed layer, comprisinggenerating a first seismic wave at a generating point adjacent theearth's surface, measuring the relative time required for said firstwave to travel from said generating point to each of two receptionpoints substantially collinear therewith, at least one of said receptionpoints lying above the bottom of said weathered layer, generating asecondseismic wave at one of said reception points, and measuring thetime required for said second seismic wave to travel from said onereception point to the other of said reception points, all said wavesfor which said time measurements are made having included as parts oftheir travel paths portions lying within said high speed layersubstantially parallel to the interface between said high speed layerand said low speed layer.

' HUGH C. SCHAEFFER.

RAYMOND A. PETERSON.

